A key telephone system is one which has multiple stations having access to one line and has multiple lines terminating at each station of the group. Each station instrument or subset has a line or pick-up key, one per line which may be individually depressed to allow that particular telephone line to be accessed by the station for use. When the line is in use, a direct current flow in that line, and a visual indication is usually presented of the busy condition of the line.
In the usual key system, when a line is in use, any other station may access that line and join or listen in on the conversation. To prevent this free access and to provide a privacy option, a relay and logic circuitry may be provided in such a manner to deny entry into a busy line of the key telephone system. One well-known approach is to sense the condition of the A lead between the line circuit and the key station and to provide the access denial responsive to the sensed condition.
When the line circuit is in the idle, ring up, or hold condition or mode of operation, the well-known A lead of the key telephone line circuit will have a negative voltage of approximately -24 volts D.C. on it. When the line circuit is busy, the A lead of the key telephone line circuit will have a very low negative voltage or approximately ground on it, the ground usually being drived from a switch at the station instrument. When a voltage of -24 volts is present on the A lead of the line circuit, a key telephone station with a privacy circuit installed can access that line circuit, but when ground is present on the A lead of the line circuit, it cannot be accessed by the key telephone station. Once a line circuit is accessed by a station, that station applies a ground to the A lead of the line circuit. Thus, privacy is established using the condition of the A lead circuit for initiation of the control circuit.
The privacy circuits as presently constituted utilize two relays and/or changes to the mechanical structure of the key telephone. One relay is used in one known system; however, a special phone must be used to provide this feature.